I had a situation come up where I had made a truce with a much weaker player (weaker as in he hardly had any territories left while most other players had many) and I stuck to it for several turns until it got to the point where another player attacked him and reduced him to just 3 territories, which were all on my border.

It seemed like if I continued to follow our truce, in 1-2 turns he was going to get eliminated by my opponent, who would then cash in the cards he won from the elimination to attack me.

So, I decided to eliminate the guy myself and get his cards before my opponent could.

Now, the eliminated player hasn't complained or anything, but I was just wondering if this is seen as the proper thing to do, or if I should have been expected to stick with the truce we had. I wouldn't have attacked him at all if not for being able to eliminate him and get his cards... so what say you?

It's fine. I had to do this as well and I don't think that there were any hard feelings since he was dead anyway with only a few troops left. In my case, whoever took him out was going to win that 7 or 8 player game.